Breadcrumbs are displayed by default in the header section (see
Layout and sections). With the default main template,
the header section is composed by the logo, the application name,
breadcrumbs and, at the most right, the login box. Breadcrumbs are
displayed just next to the application name, thus they begin with a
separator.

Here is the header section of the CubicWeb’s forge:

There are three breadcrumbs components defined in
cubicweb.web.views.ibreadcrumbs:

BreadCrumbEntityVComponent: displayed for a result set with one line
if the entity is adaptable to IBreadCrumbsAdapter.

BreadCrumbETypeVComponent: displayed for a result set with more than
one line, but with all entities of the same type which can adapt to
IBreadCrumbsAdapter.

The IBreadCrumbsAdapter adapter is defined in the
cubicweb.web.views.ibreadcrumbs module. It specifies that an
entity which implements this interface must have a breadcrumbs and
a parent_entity method. A default implementation for each is
provided. This implementation expoits the ITreeAdapter.

Note

Redefining the breadcrumbs is the hammer way to do it. Another way
is to define an ITreeAdapter adapter on an entity type. If
available, it will be used to compute breadcrumbs.

the main view is given as argument so breadcrumbs may vary according to
displayed view (may be None). When recursing on a parent entity, the
recurs argument should be a set of already traversed nodes (infinite
loop safety belt).

If the breadcrumbs method return a list of entities, the
cubicweb.web.views.ibreadcrumbs.BreadCrumbView is used to display
the elements.

By default, for any entity, if recurs=True, breadcrumbs method returns
a list of entities, else a list of a simple string.

In order to see a hierarchical breadcrumbs, entities must have a
parent method which returns the parent entity. By default this
method doesn’t exist on entity, given that it can not be guessed.